1. Field of the Invention
The present invention relates to a method of forming an oxide superconducting thin film, and more specifically to a novel method of forming on a silicon substrate a superconducting thin film of a Cu-oxide type compound oxide such as Y type compound oxide with high controllability of a crystal orientation.
2. Description of Related Art
Superconducting phenomenon had been considered to be phenomenon inherent to a cryogenic temperature indispensably requiring a cooling by a liquid helium. However, in 1986, Bednorz and Muller reported (La, Ba).sub.2 CuO.sub.4 showing a superconducting state at a temperature of 30 K. In 1987, Chu reported YBa.sub.2 Cu.sub.3 O.sub.y having a superconducting critical temperature on the order of 90 K., and in 1988, Maeda reported a so-call bismuth (Bi) type compound oxide superconducting material having a superconducting critical temperature exceeding 100 K. These compound oxide superconducting materials can obtain a superconducting with cooling using an inexpensive liquid nitrogen. As a result, possibility of actual application of the superconductor technology has become discussed and studied. In addition, at a beginning, the compound oxide superconducting material had been formed as a sintered body by a solid phase reaction process, but have now become possible to be formed in the form of a thin film. As a result, it has become gradually possible to obtain an increasing quality.
In application of the oxide superconducting thin film in a field of electronics, it is required to deposit a thin film having particularly a lined-up or uniform crystal orientation. Namely, it is known that the oxide superconducting material has remarkable crystal anisotropy in its superconducting properties. Therefore, at the time of depositing the oxide superconducting thin film, it is necessary to control the crystal orientation in accordance with application purpose of the deposited oxide superconducting thin film.
For example, in the case of utilizing the oxide superconducting thin film as a superconducting current path allowing a superconducting current to flow in parallel to the deposition surface of the substrate, it is necessary to deposit the oxide superconducting thin film having a "c-axis" of the crystal perpendicular to the deposition surface of the substrate. Namely, the oxide superconducting thin film is required to be a [001] oriented thin film. On the other hand, in the case of utilizing the oxide superconducting thin film in a stacked SIS (superconductor-insulator-superconductor) Josephson device, it is required to cause the superconducting current to flow perpendicularly to the deposition surface of the substrate. In this case, accordingly, it is necessary to deposit the oxide superconducting thin film having a "c-axis" of the crystal in parallel to the deposition surface of the substrate, namely, the oxide superconducting thin film oriented either in a [100] orientation or a [110] orientation.
On the other hand, the compound oxide superconducting material generally has a complicated crystalline structure, and only when it is deposited on a substrate of a special material under a special condition, a desired specific crystal orientation can be obtained. For example, as a substrate on which an oxide superconducting thin film is deposited, it is necessary to select a substrate material which matches in lattice with the oxide superconducting material and has less mutual diffusion to the oxide superconducting material. Specifically, a (100) plane of a MgO single crystal, and a (001) plane or a (110) plane of a SrTiO.sub.3 single crystal, etc. can be used.
However, the above mentioned oxide single crystal substrate is generally high in cost and little in the amount of supply. Therefore, this is one demerit for actual application of the oxide superconductor. In addition, a large size of oxide single crystal substrate is very difficult to obtain, and therefore, cannot be said to be a suitable material if an inclination of a large scale of the oxide superconducting thin film is considered. Under these circumstances, it has been proposed to use, as a substrate for formation of the oxide superconducting thin film, a silicon wafer which are an inexpensive and high quality substrate material most stably available at present.
However, if the silicon substrate is used as a substrate for formation of the oxide superconducting thin film, the oxide superconducting thin film reacts with a silicon (Si) of the substrate material, with the result that the formed oxide superconducting thin film can have only a remarkably deteriorated superconduction property, or has lost the superconducting property.
In order to prevent the oxide superconducting thin film from reacting with the substrate material, it has also been proposed to form a buffer layer on a deposition surface of the substrate, and then to deposit the oxide superconducting thin film on the buffer layer. However, there has not yet been known a high quality buffer layer which permits control of the crystal orientation of the oxide superconducting thin film.